This application is based on Application No. HEI 9-267036 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an organic electroluminescent element.
2. Description of the Related Art
Organic electroluminescent elements are elements constructed using organic compounds as luminescent materials which emit light in response to electric signals.
Organic electroluminescent elements have a basic structure of an organic luminescent layer interposed between a pair of opposing electrodes. Electroluminescence is a phenomenon wherein electrons are injected from one electrode and holes are injected from another electrode so as to excite an illuminant within the luminescent layer to a higher energy level, and excess energy is discharged as light when the illuminant returns to its original base state.
In addition to the aforesaid basic structure, a hole injecting layer is added to the electrode which injects holes and an electron transporting layer is added to the electrode injecting electrons so as to improve luminance efficiency.
An example of an organic electoluminescent element is disclosed in U.S. Pat. No. 3,530,325, which describes an electroluminescent element using a monocrystal anthracene as a luminant.
U.S. Pat. No. 4,539,507 discloses an organic electroluminescent element combining a hole injecting layer and an organic electroluminescent layer.
U.S. Pat. No. 4,720,432 discloses an organic electroluminescent element combining an organic hole injecting layer and an organic electron injecting layer.
These electroluminescent elements with multi-layer structures comprise multiple layers of an organic fluorescent body, charge-transporting organic material (charge-transporting member), and electrodes, wherein luminescence is accomplished by holes and electrons injected by said respective electrodes moving through said charge-transporting member and again coupling. Examples of organic fluorescent bodies include organic colorants which fluoresce such as 8-quinolinol aluminum complex. Examples of charge-transporting materials include N,Nxe2x80x2-di(m-tolyl)N,Nxe2x80x2-diphenylbenzidene, diamino compounds such as 1,1-bis[N,N-di(p-tolyl)aminophenyl]cyclohexane and the like, and 4-(N,N-diphenyl)aminobenzaldehyde-N,N-diphenylhydrazone compounds and the like. Porphyrin compounds such as copper phthalocyanine have also been proposed.
Although organic electroluminescent elements have high luminance characteristics, they also are unstable when luminescing and have poor stability during storage so as to be impractical for use. One disadvantage of the aforesaid elements regarding storage stability and stability during luminescence pertains to the stability of the charge-transporting material. The layers of the electroluminescent element formed of organic material are quite thin at 100 to several hundred nanometers, and an extremely high voltage is applied to the layer per unit thickness. Heat is generated by luminescence and current flow, such that electrical, thermal, and chemical stability is required by the charge-transporting material.
Japanese Laid-Open Patent Nos. HEI 2-15595, 3-37994, 4-132191, and 5-121172 disclose elements which replace the conventionally used aluminum with a negative electrode to reduce the luminescence starting voltage of the organic electroluminescent element.
Further disadvantages arise, however when metals other than aluminum are used, inasmuch as the layer formation conditions become more difficult, oxidation may occur during layer formation, black spots become prevalent when luminescing.
In light of the aforesaid information, an object of the present invention is to provide an organic electroluminescent element which possesses increased luminescent intensity and exhibits stable characteristics even with repeated use.
The present invention relates to an organic electroluminescent element having at least a positive electrode, luminescent layer, and negative electrode, wherein said negative electrode is a mixed layer of a plurality of metals having different work functions, and a higher percentage of metals having high work function is greater on the exterior side of said mixed layer.